The present invention relates to a sheet feeder applicable to various kinds of sheet processing apparatuses and, more particularly, to a sheet feeder incorporated in a laser printer, copier, facsimile transmitter or similar image forming apparatus for feeding cut sheets continuously.
An image forming apparatus of the kind described includes a sheet feeder for feeding a sheet to record an image formed on a photocnductive element or similar image carrier. Usually, the sheet feeder has a sheet cassette loaded with a stack of sheets, a pick-up roller for picking up the uppermost sheet of the stack, and a grip roller pair to which the sheet picked up is driven via a feed roller and a separation roller facing each other. The sheet transported by the grip roller abuts against a register roller pair with the leading edge thereof sensed by a register sensor. As a result, the movement of the sheet is stopped for a moment to synchronize the sheet to the start of image formation. On the start of image formation, the register roller pair starts rotating to thereby drive the sheet to an image transfer section. Consequently, an image is transferred from the photoconductive element to the sheet. Such a conventional sheet feeder transports a sheet at a substantially constant speed at all times in synchronism with the image forming speed of the apparatus. On the other hand, a sheet feeder capable of changing the transport speed is disclosed in, for example, Japanese Patent Laid-Open Publication No. 236131/1989. In this type of sheet feeder, even when the interval or distance between consecutive sheets changes due to, for example, the irregular positions of sheets in the stack or the slippage occurred in the event of sheet transfer, the transport speed of the following sheet is increased to compensate for the increase in the interval. This is successful in eliminating a sheet jam ascribable to a change in the interval between sheets.
With a conventional analog copier, it is necessary to scan a document every time an image is to be formed. This, of course, requires a substantial distance between the preceding and succeeding sheets. Regarding a page printer or a facsimile printer using plain sheets, reducing the distance between consecutive sheets, has not been discussed much since an image processing time is needed before the next printing. However, there is an increasing demand for an implementation capable of reducing the interval between sheets as far as possible to cope with a digital copier and a high speed and efficient image forming device incorporated in a printer or a facsimile.
This demand, however, cannot be met by the conventional sheet feeder. Specifically, the preceding sheet is temporarily brought to a stop by the register roller pair after it has been fully transported and is driven again in synchronism with the image formation. This kind of control is not practicable unless a sufficient interval is provided between consecutive sheets. In addition, since the sheets are each abutted against the register roller pair, the distance between the trailing edge of the preceding sheet and the leading edge of the following sheet increases, i.e., it becomes greater than at the time of the start of sheet feed. Moreover, while the sheets are transported from the stack to the register roller pair, the distance between them is noticeably effected by the irregular positions in the stack, the irregular rotation speeds and aging of the rollers, the irregular positions of the rollers, the slippage of the sheets on the rollers, the deformation of the sheets on the transport path, the error of the sensor, etc. Hence, it is necessary to provide a distance between sheets great enough to compensate for such irregularities at the start of sheet feed. More specifically, since images are continuously formed with a sufficient interval matching the irregularities and the registration time provided between consecutive sheets, an interval as great as 30% to 50% of the sheet length is simply wasted, as measured at the image forming section.
The problems described above are also true with the conventional sheet feeder disclosed in the above-mentioned Japanese Patent Laid-Open Publication No. 236131/1989. Specifically, when the interval between sheets is increased due to the irregularities, the sheet feeder accelerates the rotation of a control roller and other rollers for a predetermined period of time to thereby correct, i.e., reduce the interval. This maintains the interval between sheets constant and allows sheets to be fed with accuracy. However, the sheet feeder causes the sheet having been corrected to abut against the register roller remaining in a stop so as to position the leading edge of the sheet and correct skew, and then drives it again in synchronism with an image formed at the image forming section. As a result, the interval between the sheets sequentially transported to the image forming section is great.
In the light of the above, Japanese Patent Laid-Open Publication No. 8756/1988, for example, proposes an arrangement wherein two pairs of register rollers and transport paths each being associated with one of the two roller pairs are provided. The register roller pairs and the transport paths are switched over to reproduce images continuously without providing an interval between consecutive sheets. Although this approach is capable of surely transporting sheets with conventional control accuracy, it needs a complicated mechanism for selectively feeding a sheet to either of the two register roller pairs. This not only increases the cost but also makes the sheet feeder bulky and unreliable. Further, Japanese Patent Laid-Open Publication No. 130944/1987 teaches a sheet feeder having at least one continuously operable sheet transporting means between sheet separating and feeding means and a photoconductive element. The sheet transporting means is provided with a relatively low transport speed, so that sheets continuously fed from a stack without any interval may be spaced apart by an adequate distance at an image forming section. This type of sheet feeder synchronizes a sheet and an image formed on a photoconductive element by using a roller rotating continuously and an output of a sensor in place of the abutment of the leading edge of a sheet against a register roller pair. However, the problem with such a scheme is that the sheet separating and feeding means cannot operate stably due to, for example, changes in the positions of individual sheets of a stack, the separating force acting on the sheets, and the friction acting on the sheets. This makes it difficult to feed sheets continuously without providing a substantial distance therebetween. Any irregularity occurring in the sheet separating and feeding means directly translates into a dislocation of an image relative to a sheet, degrading the image quality.